| The traditional energy crisis and environmental pollution drive the rapid development of emerging techniques,among which photocatalysis has demonstrated obvious advantages by utilizing solar light as the energy source.The key to realizing highefficiency photocatalytic conversion is to design and synthesize solar-driven photocatalysts.Compared with the wide-bandgap semiconductors,the narrow-bandgap semiconductors,especially the two-dimensional ones that could absorb visible light,possess abundant surface active sites and short electron transfer distance,have attracted comprehensive attention.However,most of the narrow-bandgap oxide semiconductors have a positive conduction band,resulting in a short photogenerated electron lifetime and hence high charge recombination.Therefore,it is critical to prolong the lifetime of the photogenerated electrons by developing rational strategies.Constructing heterojunction is generally accepted as an effective strategy for enhancing charge separation.Therefore,coupling wide bandgap semiconductors with a proper conduction band has been put forward to be an effective approach.In this work,the traditional wide-bandgap semiconductor SnO2 with the suitable conduction band was selected as the electron platform to couple with the narrow-bandgap semiconductors,including the classical twodimension g-C3N4,α-Fe2O3 and the novel Bi3.2Mo0.8O7.5.The visible-light-generated electrons of the narrow-bandgap semiconductors would transfer to the conduction band of SnO2,resulting in the photoelectrons with prolonged lifetime and maintained thermodynamic.Moreover,the nanosized Ag was further introduced on the SnO2 as cocatalyst in the narrow-bandgap semiconductor-based nanocomposites.Note worthily,SnO2 as the electron platform and Ag cocatalyst could demonstrate synergistically improving effect on the visible-light photoactivity.Based above,three individual works were accomplished and shown below.I.Co-modification of nanosized SnO2 and Ag on g-C3N4 nanosheets to improve the visible-light activities.In this work,nanosized SnO2 was in-situ modified onto the urea-derived g-C3N4 nanosheets by the microwave-assisted hydrothermal method.Subsequently,nanosized Ag and Au were selectively loaded on SnO2 in the SnO2/g-C3N4 nanocomposites by the photo-deposition method.The optimal ternary Ag-SnO2/g-C3N4 nanocomposite displayed～10-and～8-time photoactivities compared with pristine gC3N4 nanosheets for visible-light CO2 reduction and aerobic 2,4-dichlorophenol degradation,respectively.Based on the transient-state surface photovoltage responses,transient-state photoluminescence spectra,and electrochemical analyses,it is demonstrated that the photoelectrons of g-C3N4 were transferred to the SnO2 platform under visible-light irradiation to prolong the photoelectron lifetime and enhance the charge separation,and the nanosized Ag synergistically worked with the SnO2 platform to further enhance the charge separation and promote the activation of CO2 and O2,resulting in the improved photoactivities.The photoactivities of Au-SnO2/g-C3N4 were lower than those of Ag-SnO2/g-C3N4 because the synergistic effect of Au is weaker than that of Ag due to the poor activation capacity.Ⅱ.Construction of Ag-SnO2/α-Fe2O3 nanocomposite photocatalysts for improved visible-light activities.Firstly,α-Fe2O3 nanosheets were synthesized by a metal-ion-intervened hydrothermal method.Secondly,SnO2 nanosheets were coupled with α-Fe2O3 nanosheets to obtain SnO2/α-Fe2O3 nanocomposites.Finally,nanosized Ag was selectively loaded on SO by the photo-deposition method to prepare the ternary AgSnO2/α-Fe2O3 nanocomposites.The optimized Ag-SnO2/α-Fe2O3 nanocomposite could realize efficient aerobic degradation of 2,4-dichlorophenol as a representative organic pollutant under visible-light irradiation(>420 nm),exhibiting nearly 6-fold degradation rates of those for the α-Fe2O3 nanosheets.Besides,the Ag-SnO2/α-Fe2O3 photocatalyst is also applicable for the visible-light degradation of other organic pollutants and even CO2 reduction.By using steady-state surface photovoltage spectroscopy,fluorescence spectroscopy,and electrochemical methods,the photoactivity enhancement of AgSnO2/α-Fe2O3 is principally attributed to the enhanced charge separation achieved by introducing SnO2 as an electron platform for the high-level-energy electrons of α-Fe2O3.Moreover,nanosized Ag as a cocatalyst synergistically enhanced the charge separation by facilitating catalytic reduction.Ⅲ.Construction of SnO2-Bi3.2Mo0.8O7.5 nanocomposite photocatalysts for improved visible-light activities.A novel two-dimension Bi3.2Mo0.8O7.5 narrow-bandgap semiconductor with a thickness of around 4 nm was synthesized by adjusting the molar ratio between Bi and Mo via a microwave-assisted hydrothermal method.Compared with common Bi2MoO6,Bi3.2Mo0.8O7.5 nanosheets displayed improved visible-light activities due to the unique surface chemical structure and two-dimension morphology.A subsequent modification of SnO2 nanoparticles on the surface of Bi3.2Mo0.8O7.5 nanosheets by an electrostatic-interaction-induced assembly process resulted in the SnO2Bi3.2Mo0.8O7.5 nanocomposites.The optimized SnO2-Bi3.2MoO.8O7.5 nanocomposite exhibited-2-time visible-light activity for the aerobic 2-chlorophenol degradation compared with pristine Bi3.2Mo0.8O7.5.The fluorescence spectra related to hydroxyl radicals and normalized photocurrent action spectra indicated the introduction of SnO2 nanoparticles as an electron platform facilitated the charge transfer and separation.The scavenger experiments revealed the photogenerated holes were the dominant active species for inducing degradation.In addition,the SnO2-Bi3.2Mo0.8O7.5 photocatalyst was also applicable for CO2 reduction. |
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